They can get dehydrated, suffer from heat stroke, and if left long enough, die.
Well since you are asking a question like this I don't know if having a child is really smart, with all the car jackers it's just not smart to leave precious valuables in the car :)
no specific age, but if your planning on leaving your child in a car, your an awful parent
Pain or death for the child.
Unfortunately, the parent (s) of the child are liable for the damages to the car, since they're fully responsible for the child's actions.
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Kaitlyn's Law is the law governing the care and safety of children with respect to leaving them in the car. Basically, as long as the car is not running, doors are locked, and the car properly ventilated, providing the child is otherwise safe, a child can be left in a car unattended if age 6 or older. Under the age of 6, the child must be accompanied by a child age 12 or older, when the car is left running.
If your child has a license the insurance on the car will probably cover it. The company can take the position that they were not supposed to be driving it and are not covered in which case it would come back to you because the child is underage.
Nothing except maybe getting the owner of the car arrested.
Cars parked in direct sunshine can get real hot. And small kids can overheat easily. If it gets real bad they can get permanent injuries and even die.
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NO AGE !! NEVER leave anyone in a car on there own
A child should never be left alone in a car at any age. The state of California, as well as all other US states, make this a crime punishable by jail time if a child is left in a car and dies.